During sleep, humans usually pass through five stages or phases of sleep: stages 1, 2, 3, 4, and REM (rapid eye movement) sleep, then the cycle starts over again with stage 1. For adults, typically almost 50 percent of total sleep time is stage 2 sleep, about 20 percent is REM sleep, and the remaining 30 percent is stage 1, 3, and/or 4 sleep. Infants, by contrast, spend about half of their sleep time in REM sleep.
Stage 1 sleep is light sleep. A person in stage 1 sleep drifts in and out of sleep and can be awakened easily. Eye movement and muscle activity are slow. People awakened from stage 1 sleep often remember fragmented visual images. Many also experience sudden muscle contractions called hypnic myoclonia, often preceded by a sensation of starting to fall.
These sudden movements are similar to the “jump” people make when startled. During stage 2 sleep, eye movements stop and brain waves (fluctuations of electrical activity that can be measured by electrodes) become slower, with occasional bursts of rapid waves called sleep spindles. In stage 3 sleep, extremely slow brain waves called delta waves begin to appear, interspersed with smaller, faster waves. By stage 4, the brain produces delta waves almost exclusively. It is very difficult to wake someone during stages 3 and 4, which together are called deep sleep. There is no eye movement or muscle activity during stage 3 or 4 sleep. People awakened during deep sleep do not adjust immediately and often feel groggy and disoriented for several minutes after they wake up. During REM sleep, breathing becomes more rapid, irregular, and shallow, significant eye movement occurs, heart rate and blood pressure increase, and limb muscles become temporarily paralyzed. When people awaken during REM sleep, they often describe bizarre and illogical tales (i.e., dreams).
The first REM sleep period usually occurs about 70 to 90 minutes after the beginning of a night's sleep. A complete sleep cycle takes 90 to 110 minutes on average. The first sleep cycles each night contain relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep periods decrease in length. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.
People lose some of the ability to regulate their body temperature during REM, so abnormally hot or cold temperatures in the environment can disrupt this stage of sleep. If a person's REM sleep is disrupted one night, the normal sleep cycle progression is often not followed the next night. Instead, such a person often slips directly into REM sleep and goes through extended periods of REM sleep to “catch up” on this stage of sleep.
Circadian rhythms are regular changes in mental and physical characteristics that occur in the course of a day. Most circadian rhythms are controlled by the body's biological “clock.” This clock, called the suprachiasmatic nucleus (SCN) is actually a pair of pinhead-sized brain structures that together contain about 20,000 neurons. The SCN rests in a part of the brain called the hypothalamus, just above the point where the optic nerves cross. Light that reaches photoreceptors in the retina creates signals that travel along the optic nerve to the SCN.
Signals from the SCN travel to several brain regions, including the pineal gland, which responds to light-induced signals by switching off production of the hormone melatonin. The body's level of melatonin normally increases after darkness falls, making people feel drowsy. The SCN also governs functions that are synchronized with the sleep/wake cycle, including body temperature, hormone secretion, urine production, and changes in blood pressure.
From experiments where people are deprived of light and other external time cues, it is apparent that most people's biological clocks work on a 25-hour cycle rather than a 24-hour one. But because sunlight or other bright lights can reset the SCN, human biological cycles normally follow the 24-hour cycle of the sun, rather than the innate 25-hour cycle. Circadian rhythms can be affected to some degree by almost any kind of external time cue, such as the beeping of an alarm clock, the clatter of a garbage truck, or the timing of meals.
When travelers pass from one time zone to another, they suffer from disrupted circadian rhythms, an uncomfortable feeling known as jet lag. For instance, a person traveling from California to New York will “lose” 3 hours according to his or her biological clock. Such a traveler will feel tired when the alarm rings at 8 AM the next morning because it is still 5 AM according to the traveler's biological clock. It usually takes several days for a traveler's body cycles to adjust to the new time.
To reduce the effects of jet lag, the biological clock can be manipulated with a technique called light therapy. People are exposed to special lights, many times brighter than ordinary household light, for several hours near the time the subjects want to wake up. This helps them reset their biological clocks and adjust to a new time zone. See a book entitled “Promise of Sleep” by Dr. William Dement (pgs. 92-96, 408).
Although insufficient sleep is a common problem, there is also research that shows that people that sleep more than 8 hours a day have a higher mortality rate than those that sleep less than 8 hours. Further studies show that a healthy range for a night's sleep can be as short as 4 hours for some individuals. While people typically think they are tired because of insufficient sleep, people may in fact be tired or shortening their lives because of too much sleep. See for instance, “Mortality Risk Associated with Sleeping Patterns of Adults” by Deborah L Wingard and Lisa F Berkman in the Feb. 15, 2002 issue of the Archives of General Psychiatry.
As indicated above, human sleeping behavior is complex, and a disruption of a normal sleeping pattern (as in jet lag), or an undiagnosed abnormal sleeping pattern, can have significant adverse health and/or performance consequences. For this reason, methods for monitoring sleep to account for the difference between various stages of the sleep cycle have been considered in the art. For example, U.S. Pat. No. 4,228,806 considers an alarm clock having a monitor to determine whether a user is in a deep sleep stage or not. An alarm interval is set, and the alarm sounds at the first time during the interval when the user is not in a deep sleep stage or at the end of the interval if the user is in deep sleep throughout the interval. Similarly, US patent application publication 2002/0080035 considers an alarm clock having an alarm that is automatically adjusted to account for a user's sleep stage at the time of awakening (e.g., a relatively loud alarm is sounded if the user is in deep sleep, and a relatively soft alarm is sounded if the user is in light sleep).
However, these methods do not enable a user to fully optimize and control his or her sleep cycles to improve health and/or performance. Thus, there is an unmet need in the art for such methods and systems.